The present application claims priority to Czech Republic Patent Application PV 2016-6, filed Jan. 7, 2016, which is incorporated by reference. The technical solution concerns a method for accurate automated non-invasive measurement of blood pressure waveform and the apparatus to carry out the method. By using the device and the method, and based on measured signals processed by developed algorithms it is possible to effectively determine important hemodynamic parameters of cardiovascular system. This device is intended for use in human and veterinary medicine, especially in the investigation of the state of cardiovascular system, i.e. hemodynamic parameters of bloodstream.
Devices for determination of hemodynamic parameters of cardiovascular system based on measurement of pressure waves are widely used in human medicine. They enable to determine many advanced hemodynamic parameters that enable complex assessment of cardiovascular system including pulse wave velocity (PWV), augmentation index (AIx) that servers as basic parameters for assessment of Arterial Stillness (AS) and risk of development and progression of arteriosclerosis and associated comorbidities.
Superior and most precise method for assessment of above-mentioned parameters still remains invasive measurement by catheter introduced to aortic root. This is still invasive operating procedure associated with certain known health risks and high price. The examination can't be performed under ambulatory conditions and have to be performed by certified interventional cardiologist in specially equipped operating room for angiology including catheter and other equipment to perform this type of surgery.
Non-invasive substitution of the invasive method is described in patent U.S. Pat. No. 6,117,087 and uses contact pressure sensor and sophisticated mathematical models derived from limited amount of data measured of patients with indications of several cardiovascular diseases. Learning models under such circumstances introduces high infidelity into the method.
Another solution is described in Czech patent number 295119, where measurement of blood pressure pulse wave is accomplished by contact pressure sensor connected to differential pressure sensor that increases sensitivity of measured pressure curve. This solution enables to measure very small blood pressure pulse waves from the surface of pressurized arteries at radial arteries with option of PWV analysis using special setup of device and utilization of two sensors, while there is no constriction of measured artery by the device. Placement of the device at radial artery is main limitation of the method to assess central aortic pulse wave as well as brachial pulse wave. This is because of known physiological phenomenon that describes change of pattern and amplitude of pulse wave in a particular place if measured from the aortic root distally along arm compare to pulse wave measured in the aortic root. This phenomenon applied to the method suggests that the method has substantial inaccuracy as well.
Main disadvantages of devices that use contact pressure sensor (e.g. Sphygmocor CVMS by AtCor) are changes of sensed pulse wave that are introduced by movements of examiner (physician) and by patient during measurement. These changes generate inaccuracies. Another main disadvantage of the method is that measurement can't be carried out automatically and it needs trained medical personnel.
SphygmoCor XCEL (AtCor), Arteriograph (TensioMed) BP+ (Uscom) that determine hemodynamic parameters of cardiovascular system from pattern of suprasystolic pressure pulse waves doesn't offer solution of above-mentioned disadvantages. Measurement is carried out automatically and by using arm cuff for measurement of suprasystolic pressure pulse waves. As described in patent WO 2005077265 A1, these pulsations are sensed using suprasystolic pressure (preferred is a systolic pressure +35 mmHg) by standard pressure sensor used in devices for blood pressure measurement by standard oscilometric principle. Typical dynamic range of these sensors is usually min 40 kPa, and relative overpressure ˜300 mmHg. Because of small amplitudes of suprasystolic pressure pulse waves device in patent WO 2005077265 A1 uses compensation filter (“reverse filter”) that a circuit that compensates for pattern of the signal during measurement. This is method is burdened by inaccuracy and under discussion in scientific press—e.g. discussed in Validation of the Arteriograph working principle: questions still remain. Bram Trachet et al. Journal of Hypertension, 29:619-622, 2011. This remains similar also for patents US 2014135632 A1 and US 2010256507A1. Another disadvantage of above-mentioned devices is a complexity of mathematical models for assessment of hemodynamic parameters. These models are derived from limited set of the data and highly multivariate. They also need certain high amount of input parameters they use together with measured pressure pulse wave to be able to calculate a result. Other several sensors often supply these input parameters. Used models were often tested using limited sizes of study populations and limited types of cardiovascular diseases, which can introduce certain instabilities and inaccuracies into models and thus into the whole measurement system.
In an aspect of the present invention, accurate automated non-invasive measurement of blood pressure waveform according to the invention is provided. Its essence consists in the following method of measurement and in the following apparatus embodiment. The device consists of an electromechanical pump connected to an arm cuff, a differential pressure sensor, a pressure sensor A, a pressure sensor B, a control (decompression) valve, a closing valve and an air reservoir, with a minimal volume of 50 ml. Arm cuff is wrapped around the arm of the examined person and just tightened. Actual values of the pressures in the pneumatic components of the apparatus are converted into an electrical signal by the pressure sensor A, the pressure sensor B and the differential pressure sensor. These signals are then filtered using a set of passive RC low pass filters to remove high frequency interference, and thereafter through analog to digital converter (at least 12-bit) they are digitized and ready for further computer processing. The signal from pressure sensor A is after filtration amplified by an amplifier, approximately 50 to 100 times. The sampling frequency of the signal is at least 200 Hz. The control element, based on the current sensed values from the pressure sensor A and by casing the electromechanical pump, and the control and closing valves, controls and monitors the course of the cuff inflation during measurement. The control element (e.g. a microprocessor) senses, processes and evaluates the measured data that can be displayed directly on the display, or may be transferred to a PC for further processing.
In the first phase of measurement, the apparatus device according to the invention begins to inflate the arm cuff-bladder via electromechanical pump, at a rate controlled by the control element, e.g. a microprocessor. The cuff inflation speed is gradually slowed down so that the pneumatic system reaches a specified value of suprasystolic pressure. Through the closing valve, which is located between the inputs of the differential pressure sensor, and which is at this phase of measurement in open position, is inflated the air reservoir, which serves to minimize pressure variations in the pneumatic system. During the controlled inflation of the pneumatic system, the processor continually evaluates oscillometric pulsations obtained by digitalization of filtered and amplified signal from the pressure sensor A. Based on measured oscillometric pulsations during cuff inflation and cuff pressure signal from unamplified pressure sensor A, the value of suprasystolic pressure is determined by the control element (e.g. a microprocessor). The value of suprasystolic pressure is at least 30 mmHg higher than a systolic pressure of measured person. Pneumatic system is inflated to the suprasystolic pressure by an electromechanical pump before starting the measurement of blood pressure waveform.
A necessary condition for quality measurement of the pulse pressure wave by the apparatus according to the invention is to achieve suprasystolic pressure in the cuff placed on the arm over the brachial artery. After reaching the suprasystolic pressure and its stabilization, i.e. the pressure drop in the pressure sensors A and B is less than 1 mmHg/min, there is a signal from the processor to trigger the closing valve, i.e. the valve is closed. This separates the inputs of differential pressure sensor, i.e. separation of the static cuff pressure from pressure with superimposed pressure pulsations. At output of the differential sensor with a range of hundreds of Pa—preferred±250 Pa or ±500 Pa, i.e. approx.±1.88 mmHg, respectively±3.75 mmHg, appears blood pulse wave signals—pressure pulsations, separated from the static pressure cuff. Next phase of measurement is suprasystolic pressure pulsations measurement. During this phase, the tightness in separate parts of a pneumatic system is monitored using the signals from the pressure sensors A and B. This procedure yields a signal that is up to 100 times more sensitive compare to existing methods of suprasystolic pressure pulsations measurement. This thus eliminates disadvantages of existing devices for the automatic measurement suprasystolic pressure pulsations, in particular the need of using a compensation filter or derived complex multivariate models.
After suprasystolic pressure pulsations measurement is finished, a control element (e.g. a microprocessor) opens the closing valve and by regulation (decompression) valve gradually releases the cuff pressure, and pressure throughout the whole pneumatic system. During the controlled deflation the oscillometric pulsations from pressure sensor A are measurement and therefore it is possible to measure blood pressure—systolic, diastolic, mean arterial, using standard oscillometric method.
System automatically determines actual heart rate of the person using analysis of measured data. Other hemodynamic parameters of cardiovascular system of examined person are automatically determined from the pattern of the signal, e.g. systolic and diastolic blood pressure of each heartbeat, pulse wave velocity (PWV), augmentation index (AIx), central aortic pressure, area under the curve and the maximum pressure amplitude, which reflects the instantaneous stroke volume, and others.
The measured pressure curve is accurate and corresponds to the brachial pulse wave otherwise invasively measured by a catheter placed in patient's arm. The apparatus according to the invention is accurate, portable, and inexpensive and is usable even in individuals with cardiovascular disease, or cardiovascular system status changes. To operate the device is simple and it is easy to learn. Patient themselves are able to perform the measurement after a short learning practice period. It allows carrying out measurements in outpatient settings, such as surgery cardiologists, internists and general practitioners, but also at home environment. The whole one trial of scanning procedure takes at maximum two minutes and it does not burden or harm the patient.